1. Field of the Invention
The present invention relates to a vacuum circuit interrupter and more specifically to an improved vacuum circuit interrupter having a ceramic insulation disk having a bell-shaped casing made of copper to reduce increases in temperature and sound noise caused by eddy currents and magnetostrictive vibration, as well as to facilitate the initial assembly before the circuit interrupter is heated within a vacuum furnace for brazing.
2. Description of the Prior Art
In the conventional vacuum circuit interrupter, the bell-shaped casing is generally made of Fe-Ni-Co or Fe-Ni alloy, because it is preferable to use a metal having a thermal expansion coefficient similar to that of the alumina-group ceramic forming the insulation disk joined to the casing.
However, since there is a small difference in thermal expansion coefficient between the Fe-Ni-Co or Fe-Ni alloy which forms the casing and the ceramic which forms the insulation disk, and since thermal stress is produced when the two materials are brazed together, it is impossible to increase the wall thickness of the casing, that is the thickness of the opening end surface of the casing to which the insulation disk is joined and to increase the mechanical strength, and therefore it is necessary to absorb or reduce both thermal stress generated in the cooling process after brazing and mechanical shock generated from closing or opening the circuit, by providing a flange for the casing.
In addition, since the Fe-Ni-Co or Fe-Ni alloy used for the casing is a ferromagnetic material, the eddy currents generated by current flowing therethrough raises the temperature of the casing, thus preventing the interrupter from being used as a large-current circuit interrupter. The smaller the diameter of the casing, the greater the eddy current, and therefore it is very difficult to design a small vacuum circuit interrupter. Further, there is another serious problem such that the alternating magnetic field generated by the current of a commercial frequency flowing therethrough generates magnetostrictive vibration and thus produces resulting sound noises from the casing.
Further, since the Fe-Ni-Co alloy used for the casing is expensive, hard to work, and poor in ductility and malleability, there is another problem such that it is necessary to restrict the wall thickness and the size of the casing.
Furthermore, in the above-mentioned vacuum circuit interrupter, there is a problem such that it is very difficult to support the contacts, especially the fixed contact, when the interrupter is initially assembled during the manufacturing process, before the vacuum circuit interrupter is heated within a vacuum furnace for brazing.